WASHINGTON (CNN) -- With just one week to go before the Iowa caucuses, the Democratic presidential candidates are focusing on the Hawkeye state, working to clearly define the choices they are offering.

The Republican candidates, on the other hand, continue to keep their eyes on New Hampshire and the other early voting states that will quickly follow Iowa's January 3 caucuses.

The Democratic contest in Iowa is especially tight, with most polls showing Sens. Hillary Clinton of New York, Barack Obama of Illinois and former Sen. John Edwards in a three-way tie. Any one of three Democrats could win, CNN senior political analyst Bill Schneider said.

"The drama in the Democratic race is created by a simple fact: If Hillary Clinton is going to be stopped, it's most likely to happen in Iowa," Schneider said. "If Clinton wins Iowa, she could be unstoppable. If she loses Iowa, she may also lose New Hampshire and South Carolina. Then we've got a real race on our hands."

An American Research Group poll conducted December 20-23 had Clinton leading the field in Iowa with the support of 34 percent of likely Democratic caucus-goers, while Edwards received the support of 20 percent while Obama had 19 percent. The poll's margin of error was plus or minus 4 percentage points.

However, in what may be a sign of the race's volatility, a CNN/Opinion Research Corporation poll conducted December 14-18 showed a much tighter race, with Clinton leading with 30 percent, Obama at 28 percent and Edwards at 26 percent. The poll's margin of error was plus or minus 5 percentage points.

With so much at stake, the Democratic presidential candidates have been emphasizing messages meant not only to convince Iowans to support them, but also to motivate them to brave the winter weather and caucus for them.

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In her final pitch to Iowa voters, Clinton, as she has done throughout her presidential campaign, is emphasizing her experience with a simple message: "Time to pick a president."

On Wednesday, Clinton campaigned in Mount Pleasant, Iowa, with her husband, former President Bill Clinton, and their daughter, Chelsea.

"The entire country and even the world will be watching. I want you to ask yourself, 'Who will be the best president?' " Clinton said on Wednesday. "Who, if something happened that none of us can predict now, would be there able to respond and act on behalf of our country immediately?"

The Clinton campaign also Thursday launched a new television ad in Iowa and New Hampshire, titled "Stakes," that shows a picture of Clinton with white scrolling text saying, in part, "A nation at war... Troubles at home...America at a crossroads...Demands a leader...With a Steady Hand."

Obama, on the other hand, is calling for a break from the politics of the past, dubbing his final trip across Iowa before the caucuses the "Stand for Change" tour.

"This campaign has been premised on the idea that we have the chance -- maybe for the first time in a generation -- to finally come together and start tackling problems that George Bush may have made worse, but that were there long before George Bush ever took office," Obama said during a campaign stop in Iowa Wednesday.

And in a new television ad that began airing in Iowa last weekend, titled "Enough," Obama tells an Iowa crowd, "We gotta stop giving tax breaks to companies that are moving overseas and give those tax breaks to companies that are investing right here in Iowa. Enough is enough.

In the final days before the caucus, Edwards is emphasizing his image as a fighter for those struggling economically. The North Carolina Democrat on Thursday launched an eight-day, 38-county bus tour across Iowa titled, "America Rising: Fighting for the Middle Class."

Edwards also launched a new ad in Iowa last weekend touting his $25 billion job-creation plan. It calls for increasing federal aid to help "hard-working families across America [who] are already struggling to make ends meet," the former senator said in a statement.

The Republican contest in Iowa is coming down to a race between former Arkansas Gov. Mike Huckabee and former Massachusetts Gov. Mitt Romney, both of whom are fighting for the support of social conservatives.

An American Research Group poll conducted December 20-23 had Huckabee leading in Iowa with support of 23 percent of the likely Republican caucus-goers polled, while Romney was backed by 21 percent. Former Sen. John McCain was backed 17 percent of those polled. The poll's margin of error was plus or minus 5 percentage points.

"If Huckabee wins, we're likely to see economic and foreign policy conservatives coalesce around an alternative," CNN's Schneider said. "But who? That could be decided five days later in New Hampshire."

Having to keep an eye on the other early-voting states, the Republican candidates were scattered across the country Thursday. Huckabee was campaigning in Iowa but was planning to spend part of his day in Florida.

Romney, who had been the front-runner in Iowa and New Hampshire, planned to campaign in New Hampshire, hoping to shore up his support there after recent polls show McCain challenging his lead there. McCain on Thursday planned to campaign in Iowa.

And Giuliani, who is the Republican's national front-runner, planned to campaign in Florida, which will hold its primary on January 29. The Sunshine State could be critical to Giuliani since he is trailing in Iowa, New Hampshire, Michigan and South Carolina.

Florida could provide Giuliani with his only primary victory before "Super Duper Tuesday" on February 5. On that day 24 states -- including the delegate-rich states of California, New York, New Jersey and Illinois -- will be conducting either primaries or caucuses.

The assassination of former Pakistani Prime Minster Benazir Bhutto in Rawalpindi, Pakistan, on Thursday could add uncertainty to the already uncertain political situation in the Iowa, Schneider said. Watch how Bhutto's death could affect the race »

"Knowledge of the world and experience may become larger factors in both parties," Schneider said.

On the Democratic side, that could help Clinton, Sen. Joe Biden of Delaware, the current chairman of the Senate Foreign Relations Committee; and New Mexico Gov. Bill Richardson, who served as the U.S. ambassador to the United Nations and who has negotiated for hostages held overseas, Schneider said.

Among the Republicans, Schneider said, an increased focus on foreign affairs could help Giuliani, who had made national security and war on terrorism the keystones of his campaign, and McCain, who has argued his experience makes him ready to be commander in chief from day one.

Coincidentally, Giuliani Thursday launched a new ad that invokes his connection to events on September 11, 2001, and his tough stance on terrorism.

In the 30-second spot, titled "Freedom," Giuliani compares the firefighters he saw at Ground Zero on September 11 to the members of the "greatest generation" that fought in World War II, speaking over still images of both groups.

"During the day of September 11 living through the things that I saw and observed. Immediately, when I saw people helping each other. I saw the picture of the firefighters putting the flag up at ground zero. I said, these are the children or grandchildren or great-grandchildren of the greatest generation. They have the same resolve. The same understanding," Giuliani says in the ad.E-mail to a friend